1. Field of the Disclosure
The present disclosure relates to a casing assembly for a gas turbine engine. In particular, the present disclosure relates to a casing assembly for housing the high pressure compressor of a gas turbine engine.
2. Description of the Related Art
The present disclosure relates to a casing assembly for a gas turbine engine. In particular, the present disclosure relates to a casing assembly for housing the high pressure compressor of a gas turbine engine.
With reference to FIG. 1, a ducted fan gas turbine engine is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, a combustor 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
A double skin casing comprising a radially inner and radially outer casing surrounding the high pressure compressor 14 and combustor 15 may be provided.
The outer casing is used to support the engine carcass loads and the bearing loads from the high pressure rotor. The inner casing needs to be isolated from these engine carcass loads. The engine carcass loads cause distortion in the outer casing and it is important to avoid such distortions in the inner casing to ensure that it remains concentric about the rotor to allow tight running clearances between the high pressure compressor blade tips and the inner casing.
The inner casing is typically radially supported at the fore (upstream) and aft (downstream) ends of the high pressure rotor. The axial loads on the casings (resulting from the difference between the forward loaded high pressure compressor and the rearward loaded combustor and high pressure turbine nozzle guide vane assembly) are reacted at diametrically opposed port and starboard supports at the fore end of the inner casing, the two supports each transmitting half of the axial load as the outer casing flexes in a vertical plane. The circumferential torque acting on the inner casing is reacted by cross keyed dogs towards the aft of the inner casing.
The double skin casing design has undesirable cost and weight implications because of the high amount of material needed to form the double casing along the length of the high pressure rotor. Furthermore, distortion of the inner casing may occur around a bleed offtake which may be located at the exit of the high pressure compressor and may comprise a floating duct which passes between the two casings and which is bolted to the inner casing. This distortion affects the concentricity and roundness of the inner casing about the high pressure rotor.
Furthermore, axial loads, which may be generated for example by a turbine active tip clearance control (ATCC) system (where present), can affect the concentricity of the inner casing about the high pressure rotor. Such an ATCC system cools the outer casing which radially chocks with the inner casing, forcing it radially inwards to close the high pressure turbine tip clearance. This radial chocking prevents axial sliding between the inner and outer casings. Where the inner and outer casing expand by differing amounts (e.g. during a transient), distortion of the inner casing can occur as a result of this radial chocking.
In an alternative design, a single skin casing is provided around the combustor with a double skin casing comprising a radially inner casing and a radially outer casing around the high pressure compressor. The inner casing is bolted to the outer casing via a full circumferential flange towards the aft of the inner casing, the flange acting as both a radial and axial support. The fore of the inner casing is supported radially within a sliding birdmouth joint.
This casing design has the advantage of reduced weight and avoids the radial chocking problem associated with the ATCC system. However, in this single/double skin casing design the inner casing is not sufficiently isolated from the engine carcass loads and significant distortions can occur thus undesirably affecting the concentricity of the inner casing around the high pressure rotor and the tip clearance for the high pressure compressor. Significant distortion can occur in this design as a result of a moment created around the thrust mount (which mounts the engine to the wing pylon) which is amplified during take-off. Furthermore, the bolted flange extending between the inner and outer casings results in radial forces arising from the mismatch between the thermal properties of the material forming the two casings. Finally, machining bolt-holes for connection of the flange to the outer casing is problematical.